Note: Descriptions are shown in the official language in which they were submitted.
CA 02681282 2011-11-04
WYSSMONT 3.0-013
SYSTEM AND METHOD FOR DRYING AND TORREFACTION
BACKGROUND OF THE INVENTION
[0002] Torrefaction is generally known as the process of
thermal treatment of various cellulosic materials under an
inert (i.e., non-oxidizing) atmosphere to convert such
materials into a more useful product. For example, wood when
dried and heated to a temperature of about 200 C to 290 C
becomes what has been referred to as "torrefied wood", i.e., a
carbon-neutral product with a high fixed carbon content, a
high caloric value, a very low moisture content, and which is
generally non-hygroscopic and rot proof, having retained
practically all the pyroligneous compounds. Examples of
torrefaction and equipment therefore are disclosed in U.S.
Patent No. 4,787,917 and U.S. Patent Application Pub. No.
2003/0221363.
[0003] The present invention relates in general to
improvements in systems and methods for drying and
torrefaction of various materials.
SUMMARY OF THE INVENTION
[0004] It has been known to first dry the material to be
torrefied in one piece of equipment, and to subsequently
conduct the torrefaction in a separate piece of equipment at
higher temperatures. This has obvious disadvantages. One
aspect of the present invention is to provide a single
apparatus for both drying and torrefaction of various
1
CA 02681282 2009-09-30
WYSSMONT 3.0-013
materials in a torrefaction chamber, particularly cellulose
based materials such as wood and the like. To this end, one
aspect of the invention provides a stacked multi-level tray
heating apparatus having an upper portion or section where the
material to be torrefied is first dried. As the material is
dried, the moisture content driven off from the material is
converted into steam and/or gas, at least some of which may be
recycled into a lower portion or section of the apparatus
where the material is to be torrefied. According to another
aspect, the material may be subjected to quenching prior to
discharge from the apparatus.
[0005] By way of one example, raw wood is run through a
wood chipper to produce wood chips having a preferred particle
size of less than about h-inch. The wood chips may also
include pellets, shreds, sawdust, wood flours and the like.
The chipped wood is stored in a large hopper and is metered
from the hopper into the top of the drying chamber. No
special treatment of the wood is required prior to its
introduction into the drying chamber. The temperature of the
feed stock entering the drying chamber depends upon prevailing
climatic conditions at the time of use, and can be anywhere
from about (-)10 C to about 40 C. Moisture content of the
feedstock can be generally anywhere in the range of about 10%
to about 60% of the original weight of the feedstock. However,
it is contemplated that higher moisture contents can be
processed as well.
[0006] The feedstock is introduced into the top of the
drying zone at a uniform rate via a feeder where it is
deposited onto the uppermost tray in the torrefaction chamber.
The uppermost tray of the torrefaction chamber rotates slowly
in a horizontal plane. After being carried almost completely
2
CA 02681282 2009-09-30
WYSSMONT 3.0-013
around on the tray, the wood chips encounter a wiper bar which
plows the woods chips until they fall through one of several
slots in the tray. The chips that have been pushed off the
first tray are now deposited onto a second tray below the
first tray. This tray is also slowly rotating in
synchronization with all of the other trays in the
torrefaction chamber. After continuing to rotate a short
radial distance, the chipped wood on the second tray may
encounter a leveler bar if provided which causes the pile of
wood chips to be evenly distributed upon the tray at a
predetermined height. This predetermined height is adjustable
by way of example between about to about 3-inches.
Distributing the pile of chips at a uniform height allows all
of the chips to be uniformly warmed by the hot gas circulating
within the top of the torrefaction chamber, and this in turn
results in a very uniform processing condition.
[0007] The top section or zone of the torrefaction chamber
is temperature controlled to within about 1 C. The exact
temperature used with the top zone depends upon the species of
wood being torrefied, the initial moisture content of the wood,
and other variables related to the properties of the feedstock.
[0008] After being carried almost completely around on the
tray, the wood chips encounter a wiper bar which plows the
woods chips until they fall through one of several slots in
the tray. The chips are then deposited upon the third tray
down from the top. This process continues until the torrefied
chips are deposited upon the lowermost tray. After being
carried almost completely around on this tray, the wood chips
encounter a wiper bar which plows the woods chips until they
fall through one of several slots in the tray onto the floor
of the torrefaction chamber or directly to a discharge chute.
3
CA 02681282 2009-09-30
WYSSMONT 3.0-013
A final series of wipers push the torrefied chip out the
bottom discharge of the torrefaction chamber.
[0009] The torrefaction chamber is multi-zoned to give very
accurate drying and torrefaction of the wood chips as they
progress down through the torrefaction chamber. The present
invention has advantages of not forming a pyrochar. A
pyrochared material has a high ash content, and is less
valuable because it is not as hydrophobic, and will not
pelletize as well as torrified wood. One or more internal
fans which can be mounted vertically in the center of the
torrefaction chamber pushes the inert heated atmosphere around
in a circular, horizontal pattern within the torrefaction
chamber and across the pile of chips as they lay on the
rotating trays.
[0010] In torrefaction, various gases are evolved, such as
carbon monoxide, carbon dioxide, various organic compounds,
water, and possibly other non-organic compounds. One aspect
of this invention is to burn the carbon monoxide and organic
vapors and use the heat evolved to provide some of the heat
for drying and torrefying. The inert atmosphere may consist
substantially of steam and/or other inert gas such as nitrogen.
Torrefacation of the cellulosic products, and more
specifically the removal of bound water and volatiles in the
feedstock, are conducted in a closed, inert, system which
allows capture of volatile materials so that the commercial
value of the captured volatiles can be realized either by
combustion to recover their caloric value or by recovery as a
saleable by-product.
[0011] In accordance with one embodiment there is described an
apparatus for the torrefaction of water containing cellulosic
material within an inert atmosphere, the apparatus comprising
4
CA 02681282 2009-09-30
WYSSMONT 3.0-013
a material processing chamber having a plurality of processing
zones adapted for processing water containing cellulosic
material, a plurality of material supports within the
processing zones for receiving cellulosic material cascading
within the chamber, the material processing chamber having a
first outlet for the discharge of steam generated from
processing the cellulosic material within the processing
zones, and at least one inlet for recycling at least a portion
of the steam into the material processing chamber, whereby the
steam at least partially provides an inert atmosphere within
the material processing chamber.
[0012] In accordance with another embodiment there is
described an apparatus for processing materials, the apparatus
comprising a material processing chamber having an upper
portion and a lower portion, a rotatable assembly within the
chamber extending from the upper portion to the lower portion,
the assembly including a plurality of vertically displaced
material supports supported on a portion of the assembly; a
drive device for causing the material supports to rotate; a
first inlet at the upper portion of the material processing
chamber adapted to receive material to be processed and to
deposit the material at least partially onto at least one
material support; a first exhaust outlet for exiting exhaust
steam generated within the material processing chamber; a
second inlet for recycling at least a portion of the exhaust
steam into the material processing chamber; snd wherein said
material processing chamber is at least partially maintained
under an insert atmosphere by the recycled exhaust steam; and
a heat exchanger coupled between the first exhaust outlet and
the second inlet for heating the at least a portion of the
CA 02681282 2009-09-30
WYSSMONT 3.0-013
exhaust steam, at least a portion of the heated exhaust steam
recycled to the material processing chamber.
[0013] In accordance with another embodiment there is
described a method for torrefaction of water-containing
cellulosic material within an inert atmosphere, the method
comprising cascading cellulosic material between a plurality
of rotatable trays vertically stacked within a plurality of
processing zones provided within a material processing
chamber, heating the cellulosic material within the material
processing chamber to generate steam from the contained water
in the cellulosic material, discharging the steam generated
from the cellulosic material from the material processing
chamber, and recycling at least a portion of the steam to at
least one of the processing zones within the material
processing chamber, wherein the steam at least partially
provides an inert atmosphere within the material processing
chamber.
[0014] In accordance with another embodiment there is
described a method for torrefaction of water-containing
material within an insert atmosphere, the method comprising
passing material to be torrefied between a plurality of
material supports within a material processing chamber,
heating the material within the material processing chamber to
generate steam from the contained water within the material
being processed, exhausting the steam from the material
processing chamber, heating the steam exhausted from said
material processing chamber, returning at least a portion of
the heated steam to the material processing chamber, wherein
the steam at least partially creates an inert atmosphere
within the material processing chamber, and discharging the
torrefied material from the material processing chamber.
6
CA 02681282 2009-09-30
WYSSMONT 3.0-013
[0015] In accordance with another embodiment there is
described a method for processing material, comprising feeding
material into a material processing chamber having an upper
portion and a lower portion, and a rotatable assembly
extending between the upper portion and the lower portion
supporting a plurality of vertically displaced material
supports; applying heat within the upper portion of the
material processing chamber for drying the material;
collecting exhaust from the material processing chamber;
recycling the collected exhaust to the processing chamber;
torrefying the material within the lower portion of the
material processing chamber; and discharging the material from
the material processing chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The subject matter regarded as the invention is
particularly pointed out and distinctly claimed in the
concluding portion of the specification. The invention,
however, both as to organization and method of operation,
together with features, objects, and advantages thereof may
best be understood by reference to the following detailed
description when read with the accompanying drawings in which:
[0017] Fig. 1 is front elevational view of an apparatus for
torrefaction of materials such as wood containing cellulosic
materials in accordance with one embodiment of the present
invention;
[0018] Fig. 2 is a cross-sectional view of another embodiment
of a torrefaction apparatus in accordance with the present
invention;
[0019] Fig. 3A is a top plan view of another embodiment of a
torrefaction apparatus in accordance with the present
invention;
7
CA 02681282 2009-09-30
WYSSMONT 3.0-013
[0020] Fig. 3B is a front elevational view of the torrefaction
apparatus illustrated in Fig. 3A;
[0021] Fig. 4 is a cross-sectional view of another embodiment
of a torrefaction apparatus in accordance with the present
invention;
[0022] Fig. 5 is a schematic illustration of a torrefaction
apparatus which may be implemented in accordance with the
present invention.
DETAILED DESCRIPTION
[0023] In describing the preferred embodiments of the
invention illustrated in the drawings, specific terminology
will be used for the sake of clarity. However, the invention
is not intended to be limited to the specific terms so
selected, and it is to be understood that each specific term
includes all technical equivalents that operate in a similar
manner to accomplish a similar purpose.
[0024] Fig. 1 shows an example of an apparatus 100 for drying
and torrefying material in accordance with one embodiment of
the present invention. As shown, a hollow chamber 110 forming
the torrefaction chamber is cylindrically or polygonally
enclosed by sidewall 116 which extends around the
circumference of the chamber, a top plate 112, and a bottom
plate 114. The chamber has two sections or portions - an
upper portion 80 and a lower portion 85. Each section will
typically have multiple zones. The upper and lower portions
are contiguous with each other thereby forming essentially a
single continuous processing chamber where drying and
torrefaction take place simultaneously on a plurality of
levels or zones.
[0025] According to this aspect of the invention, drying of
the material is generally performed in the upper portion 80
8
CA 02681282 2009-09-30
WYSSMONT 3.0-013
and torrefaction is performed in the lower portion 85. In
this manner, the apparatus can operate continuously by
continuously supplying material to be processed in the upper
portion and removing continuously the torrefied material from
the lower portion. Moreover, the drying process and
torrefaction process may complement one another and in some
circumstances the torrefaction process may compensate for
inadequate drying. For example, if material passing through
the drying process is not sufficiently dried, the torrefying
process will compensate by causing the moisture content of the
material to evaporate. In this regard, the upper portion 80
and lower portion 85 may operate at substantially the same
temperature, or one portion may operate at a higher or lower
temperature relative to the other portion.
[0026] The apparatus 10 may include any of a variety of
components for transferring the material through the different
levels. For example, the apparatus may incorporate a
plurality of vertically displaced material supports such as
shelves, or trays 120. According to one embodiment, the trays
may include apertures, thereby allowing material to pass
through from one tray to a lower tray. For example, the trays
may be attached to a rotating structure , and thus may rotate
about a substantially vertical axis as the structure rotates,
with a. cantilevered device extending over the trays pushing
material through the aperture. Alternatively, the trays may
remain stationary, and the cantilevered device may sweep
across the trays to transition the material thereon.
Accordingly, the material may be transferred from a feed port
onto a first tray level, and continuously through the upper
portion 80 and the lower portion 85 via the tray levels to a
discharge port 70. For example, wiperarms may be used to
9
CA 02681282 2009-09-30
WYSSMONT 3.0-013
transfer the material from one tray level to the next tray
level below, or gyrating trays with large perforations may be
used to shake the material from one tray level down to the
next tray. According to the invention shown in Fig. 1, a
plurality of spaced apart stacked trays 120 are rotated by a
shaft 130. As will be explained in further detail below with
respect to Fig. 2, the trays 120 may include apertures for
transferring the material from one tray level 120 down to the
next tray level as they rotate while processing the material.
[0027] Material fed through the port 10 for processing may be
undried or substantially undried, that is, having different
levels of moisture content. Typically, the feed material is
wood or another cellulosic material, such as bagasse, peat,
grasses, peanut shells, etc. having various water content
(e.g., between about 10% and 60% water), but may also include
other volatiles to be evaporated in the drying process.
Cellulosic material contains lignin, which maintains the
caloric value of the torrefied end product, and allows
pelletization of the torrefied cellulosic material. It is
contemplated that non lignin containing materials such as
mosses can also be torrefied using the apparatus and methods
of the invention.
[0028] As the material is dried in the upper portion 80 of the
chamber, the moisture content from the material is evaporated
creatingwater vapor (steam) . The steam rising through the
chamber 10 may be discharged through the exhaust port 20
attached to the top plate 112. At least some of this exhaust
steam may be recycled back to the chamber 110 to provide all
or part of an inert atmosphere. For example, the exhaust port
20 may be connected to a heat exchanger 30, where the steam
can be reheated to a superheated state. The heat exchanger
CA 02681282 2011-11-04
WYSSMONT 3.0-013
may be further connected to inlets 40 and 50, for recycling
the heated steam to the upper portion 80 and/or lower portion
85 of the chamber, respectively, at adjustable ratios.
[0029] The torrefied material will catch fire if it is exposed
to the outside atmosphere before it is sufficiently cooled.
Accordingly, another aspect of the invention provides an inlet
60 for delivering a quenching substance to the lower portion
85 of the chamber for quenching the torrefied material prior
to discharge. The quenching substance may be water, or any
other inert liquid or slurry. Quenching with water is the
fastest way to do this cooling. The quenching substance may
be discharged with the torrefied material through discharge
port 70, or may be discharged separately through another
outlet (not shown), thereby enabling a constant flow of the
quenching substance to the chamber. The quenching substance
may be recycled to the chamber, and cooled through a
refrigeration device (not shown) as may be desired.
[0030] Fig. 2 shows an example of an apparatus 100 for
processing materials according to another embodiment of the
present invention. Certain aspects of the construction of the
apparatus. to be described is disclosed and described in
co-pending published application serial No. 11/975,144, filed
on October 17, 2007 and published under No. 20090100701 on
April 23, 2009. The apparatus 100 has particular application
where toxic or reactive gasses may be present or are generated
within the apparatus during torrefaction of the material. The
apparatus 100 includes a chamber 110, in this instance a
drying/torrefying chamber, wherein the materials are
processed. The apparatus 100 further includes at least one
drive assembly 160, which may power operations within the
chamber 110, though being located outside.
11
CA 02681282 2009-09-30
WYSSMONT 3.0-013
[0031] The drying/torrefying chamber is cylindrically enclosed
by sidewall 116 which extends around the circumference of the
chamber 110, a top plate 112, and a bottom plate 114. The
chamber 110 is supported on a base 174 by supports 170 and may
be connected via expansion joints 172. The expansion joints
172 enable the supports 170 to move as the chamber expands due
to, for example, increased heat therein. This reduces stress
applied to the structure of the apparatus 100.
[0032] Inside the chamber 110, the apparatus may incorporate a
set of trays 120 surrounding a set of vertically-aligned fans
on a fan shaft 130. The fans may circulate the atmosphere
inside the chamber over the material in the trays 120. The
material to be processed may be placed on the top tray level
and progressively transferred to lower tray levels. Each tray
is connected to at least one stanchion 126, wherein several
stanchions are positioned around a fan shaft 130, thereby
forming a squirrel cage. Coupled to the stanchions 126 is a
turntable 182 at the lower end of the chamber. According to
one embodiment, the turntable 182 is connected to a rotating
tray structure which surrounds the fan shaft 130. Drive gears
160 cause the turntable 182 to rotate, thereby causing the
stanchions 126 and trays 120 to revolve.
[0033] A tray wiper 122 in the nature of a cantilevered device
may be positioned above each tray 120. As each tray 120
rotates, the tray wiper 122 transfers the material to the next
tray level. A rigidly mounted leveler 125 may brush across
the top of the material placed thereon, thereby leveling the
material and exposing materials underneath the top portion to
the environment within the chamber 110. Material that is
spilled by the tray wiper 122 over the side of the tray (i.e.,
between the shaft and the rotating trays) falls onto catch
12
CA 02681282 2009-09-30
WYSSMONT 3.0-013
plate 124. This plate 124, angularly positioned with respect
to the trays 120, causes the material which is spilled off a
tray 120 above to fall into a tray 120 below. In this manner,
the material being processed cascades downwardly from the
upper tray to the lower tray.
[0034] According to one aspect, a turntable sweeper 180 may be
positioned above the turntable 182. The turntable sweeper 180
may prevent complications potentially caused by materials
falling onto the turntable 182.
[0035] As the processed material is being rotated and moved as
described above, further drying elements may be implemented
within the chamber 110. For example, several fans 140 may be
included in the chamber 110 to facilitate circulation of
heated gasses and superheated steam therein and to effect a
more even temperature profile within the chamber. The fans
140 may be connected to the fan shaft 130 by keys 146. The
fan shaft 130 may extend beyond the bearing assembly 250 and
connect to a reducer 190 at its lower end. The reducer 190
may be powered electrically, or by other sources such as
hydraulic, steam, gas, or a mechanical crank. As the reducer
190 causes the shaft 130 to rotate, fan blades 140 would in
turn rotate, thus pushing the internal environment within the
chamber across the trays 120.
[0036] The processed material may further be exposed within
the chamber 110 to gasses provided through an inlet 152. For
example, a duct may be connected to the inlet 152, and
regenerated superheated steam, heated gasses, desiccants, or
other inert, reactive, or non-reactive gasses may be provided
to the upper portion of the chamber 110 through the duct.
Similarly, a duct may be connected to inlet 154, and
regenerated superheated steam, heated air, gasses, desiccants,
13
CA 02681282 2009-09-30
WYSSMONT 3.0-013
or other inert, reactive, or non-reactive gasses may be
provided to the lower portion of the chamber 110 through the
duct. An exhaust 150 provides an outlet for the gasses which
may be recycled to the chamber 110 through the inlets 152,
154. According to one embodiment, ducts connected to the
exhaust may lead to a conditioning unit such as a heat
exchanger 30 further connected to the inlets 152, 154,
thereby allowing the gasses to be recycled through the chamber
110.
[0037] Alternatively or additionally, internal heating within
the chamber may be used. For example, in smaller units
electrical heaters may be placed within the chamber to heat
the atmosphere. In other units, U-tubes (i.e., hollow tubes
with flames inside) may be positioned within the chamber and
connected to an exhaust and a natural gas inlet port. As the
water content from the processed materials evaporates, the
fans may blow the steam across the U-tubes for reheating.
[0038] To prevent the gasses provided to the chamber 110 from
escaping, seal assemblies are placed around the shaft 132 and
near the opening 118. Shaft 130 may be formed of metal or any
variety of other materials. Further, although the apparatus
100 as described herein includes a rotating shaft 130, the
shaft 130 may be capable of other motions, such as gyrating.
[0039] Referring to the remaining figures, there is
illustrated an apparatus 200 in accordance with another
embodiment of the present invention. By way of brief
description, Fig. 3A is a top plan view of a drying/torrefying
apparatus, and Fig. 3B is a front view thereof. Fig. 4 is a
detailed view of the inner components of the drying/torrefying
apparatus. Fig. 5 is a diagrammatic view of the valves and
connections providing fluid or gaseous substances to and from
14
CA 02681282 2009-09-30
WYSSMONT 3.0-013
the chamber of the drying/torrefying apparatus and elsewhere
in the system, such as the burner.
[0040] Substantially undried cellulosic material is dried and
torrefied in the dryer/torrefier apparatus which is
constructed as one piece of equipment. The moisture
evaporated from the wood during drying and torrefying is used
as an inert medium in the lower portion 204 of the apparatus
where the wood is being torrefied. The water vapor may be
heated to a superheated state internally within the apparatus
and/or circulated via duct 220 through an external heat
exchanger 218 before being recycled back into the apparatus.
The inert super-heated steam may be supplemented by another
inert gas such as nitrogen. If the torrefied material gets
exposed to oxygen, e.g. from air, before it is sufficiently
cool upon discharge from the apparatus, it will ignite, which
would destroy its beneficial characteristics and be a safety
hazard.
[0041] This system of drying and torrefaction in a single
apparatus is more thermally efficient than drying the material
separately. It is also simpler and less expensive from the
view point of capital expenditure and operating costs.
According to one aspect, such drying and torrefaction may be
performed in a TurboDryer unit sold by WYSSMONT Corp. as may
be modified pursuant to the present invention. However, other
systems which may be used include any type of a vertical
apparatus with trays or plates or hearths that retain the
material and in which the material moves down through the
apparatus by means of arms, blades, or other such devices.
[0042] The apparatus may also incorporate a quenching section.
For example, water can be provided at the bottom of the
apparatus or in the discharge area for the processed material.
CA 02681282 2009-09-30
WYSSMONT 3.0-013
According to one aspect, water may be provided in a constant
flow to ensure that the water remains under a predetermined
temperature. For example, an additional inlet may be fed to a
lower portion of the apparatus, with an additional outlet
being placed in the lower portion as well. The inlet and
outlet may be positioned on substantially opposing sides of
the internal processing chamber 206 provided in the apparatus,
or the outlet may be the same as the discharge for the
processed materials. In this regard, cool water may be fed to
the chamber through the inlet , and discharged through the
outlet, thereby creating a steady flow of cool water.
Moreover, the discharged water may be cooled in a separate
unit, and recycled back to the inlet . The quenching is to
cool off the torrefied material quickly. If the torrefied
material gets exposed to oxygen, e.g. from air, before it is
sufficiently cool it will ignite which, would destroy its
beneficial characteristics and be a safety hazard. An
application in which the product is not quenched may be cooled
under inert gas to avoid having the hot torrefied material
start combusting.
[0043] Torrefied product can be discharged through a double
airlock with intermediate purge to prevent exposure to oxygen
before the product is reduced in temperature to below the
auto-ignition temperature of the torrified cellulosic
material.
[0044] A system is shown in Figs. 3-5 where a portion of the
exhaust 210 from the torrefaction, which has a number of
different types of exhaust gases and vapors including but not
necessarily limited to carbon monoxide, carbon dioxide,
nitrogen, water vapor, acetic acid and/or other organic
vapors, is circulated to a condenser 214 having an inlet 215
16
CA 02681282 2009-09-30
WYSSMONT 3.0-013
and an outlet 217 which removes the water and other
condensibles. The exhaust gasses could also be discharged
directly to the atmosphere if environmentally permitted, or
passed through a thermal oxidizer. From the condenser 214,
the remaining exhaust gases may be provided to a burner 212,
where it is burned with a supplementary heating system. That
is, because the excess air in the burner 212 would cause
problems (i.e., fire) in the drying/torrefying chamber, the
exhaust gases are passed through an air-to-air heat exchanger
218 for heating the remaining portion of the discharged steam
from duct 223 to be recycled. Steam generated generally
within the upper drying section of the torrefaction chamber is
exhausted via conduit 210 via a circulation fan 221. The
portion of the exhausted steam to be recycled is fed to heat
exchanger 218 where it is reheated, e.g., superheated, via the
heated gases supplied from burner 212. The superheated steam
from the heat exchanger 218 is fed to multiple sections or
zones of the torrefaction chamber to provide heat for drying
and torrefaction through one or more inlet ducts. In
addition, the superheated steam also provides the inert
atmosphere within the torrefaction chamber.
[00451 A process for drying/torrefying wood as an example will
now be described with respect to the apparatus described
above. Wood is one type of cellulosic material. Wood is a
generic name for a wide range of differing materials.
Generally, wood is divided into two major classes: hardwoods
or softwoods.
The following table gives relative compositions:
Component % mass in softwood % mass in hardwood
Cellulose 40 to 44 % 43 to 47 %
Hemicellulose 25 to 29 % 25 to 35 %
Lignin 25 to 31 % 16 to 24 %
17
CA 02681282 2009-09-30
WYSSMONT 3.0-013
Extractives 1 to 5 % 2 to 8 %
[0046] When torrefying at elevated temperature, the
hemicellulose portion of the wood will begin to decompose at
lower temperatures than the other components of wood. This
decomposition occurs rapidly in the presence of oxygen and
less rapidly when oxygen is not present.
[0047] The apparatus and method of the present invention
provides exact temperature control in each drying/torrefaction
zone which maximizes the temperature at which torrefaction can
occur, without crossing over into the temperature at which the
exothermic reaction of the hemicellulose occurs. The higher
the temperature that can be achieved while torrefying, without
causing a decomposition of the hemicellulose, produces a more
commercially valuable torrefied wood. Torrefied wood is sold
based upon its net caloric value. In operation, the preferred
temperatures in the torrefaction chamber is in the range of
about 220 C to about 280 C, and more preferably from about 260 C
to about 280 C, although temperatures in the range of about
200 C to about 300 C are contemplated.
[0048] In implementing the torrefaction process using a
rotating tray type apparatus having a plurality of stacked
trays 222 with internal circulation fans 224 on a single shaft
226 (such as describe above) , the material being processed
drops down through the stationary feed chute 228 onto the top
tray of the rotating trays. Ideally, the material falls onto
the trays uniformly. The material may be spread out using,
for example, a mounted leveler 230 to give more uniform drying
of the material on the trays by exposing materials underneath
the top portion to the environment within the chamber. The
material on the trays rotates most of the way around the
interior of the chamber forming the drying section.
18
CA 02681282 2009-09-30
WYSSMONT 3.0-013
[0049] A tray wiper 232 in the nature of a cantilevered device
may be positioned above each tray level 222. As each tray 222
rotates, the tray wiper 232 transfers the material to the next
underlying tray. The material that is spilled by the tray
wiper may fall onto a catch plate 124 such as shown in Fig. 2
or other suitable device. The plate 124, angularly positioned
with respect to the trays 222, causes the material which is
spilled off a tray above to fall onto a tray below. In this
manner, the material being processed cascades downwardly from
the top trays to the bottom trays. This action is repeated
throughout the drying section (the upper portion 80) of the
dryer/torrefier apparatus.
[0050] In the upper drying section in the preferred operation,
the water that is evaporating from the processed material is
retained in this section where it acts as an inert medium to
prevent the wood from burning at temperatures in which it
would normally combust in air. The atmosphere with the
evaporated water may also be removed from the apparatus via
conduit 210 and then recycled back into the lower torrefaction
section of the apparatus after being re-heated via burner 218
as described above. It is also contemplated that the water
vapor can be recycled back into the upper drying section 80 if
desired.
[0051] Initially the inert atmosphere can be composed of 100%
nitrogen or some other inert gas including steam. As steam
evolves from the wood chips being torrefied, the inert
atmosphere is partially or wholly displaced by evolved water
which eventually becomes a superheated steam. Since 100%
nitrogen atmosphere may be initially used during the initial
startup, for all practical purpose the torrefaction chamber
runs all of the time with superheated steam, a small amount of
19
CA 02681282 2009-09-30
WYSSMONT 3.0-013
nitrogen, and some volatiles creating the inert atmosphere.
The torrefaction chamber can optionally include a nitrogen
flooding system which will flood the torrefaction chamber with
nitrogen in the event of a high oxygen content developing
within the torrefaction chamber due to an upset condition.
This is a safety control designed to prevent fires within the
torrefaction chamber.
[0052] The purpose of the rotating internal fans 224 as
previously described is twofold. The fans immediately mix the
incoming hot steam with the steam currently within the
torrefaction chamber. For example, steam which has been
externally heated to a temperature as high as 300 C can be
introduced into the torrefaction chamber at any of several
points along the vertical height of the torrefaction chamber
in any combination. The volume of superheated steam
introduced into the torrefaction chamber at each entry point
can be separately controlled to optimize the process.
[0053] The superheated steam which is introduced into the
torrefaction chamber is immediately mixed with steam already
within the torrefaction chamber which may be and is usually at
a much lower temperature. At the top of the torrefaction
chamber the incoming feedstock temperature could be 10 C. As
it begins to dry, the water trapped within the wood is evolved.
This water vapor immediately mixes with the steam at 300 C to
yield a much lower internal drying temperature in this zone of
the torrefaction chamber. The exact temperature within the
uppermost zone of the torrefaction chamber is monitored and
controlled. Both the inlet temperature and/or the inlet flow
can be controlled to yield any zone temperature required for
optimization of the process.
CA 02681282 2009-09-30
WYSSMONT 3.0-013
[0054] The size of the top zone can be controlled by design of
the central fans 224 or by installation of baffle plates to
separate higher trays from trays lower down in the
construction of the torrefaction chamber. The uppermost zone
can consist of any number of trays, but in one example, is in
the range of three to fifteen trays. The top zone is
typically controlled at a temperature in the range of about
200 c to about 260 C range.
[0055] As the wood chips continue down through the
torrefaction chamber, the chip temperature continues to
increase. The hotter the steam used to dry the chips the more
rapid the increase in chip temperature. As the chips continue
downward through the torrefaction chamber, they may encounter
higher steam temperatures as they pass into the next
torrefaction chamber zone. This process continues through
successively hotter zones until the chips are reduced in water
content to about 1% to about 3%. At this point the chips
begin to torrify. Volatiles are driven off and additional
water is removed from the wood chips. During the downward
passage of wood chips through the torrefaction chamber as few
as one temperature zone or multiple zones (e.g., eight or more
temperature zones) may be encountered depending upon the
design of the torrefaction chamber.
[0056] The wood chips evolving their water content results in
the partial displacement of the nitrogen initially within the
torrefaction chamber during startup. Once operating, generally
without stoppage for weeks or months at a time, there remains
no significant amount of nitrogen within the torrefaction
chamber. The internal, inert atmosphere is composed of water
in the form of superheated steam, residual nitrogen, and
volatiles that have been evolved from the wood chips. It is
21
CA 02681282 2009-09-30
WYSSMONT 3.0-013
contemplated that supplemental inert gas can be added to the
torrefaction chamber to maintain an inert atmosphere.
[0057] This water vapor is discharged as superheated steam
from the torrefaction chamber at one or more points along the
vertical sides of the chamber, the top of the chamber via
conduit 210, or the bottom of the chamber. The superheated
steam can be condensed via a condenser 214 to remove a portion
of the water and/or other volatiles from the system. For
example, if the wood chips are evolving water at the rate of
20,000 pounds per hour of water, the condenser can be designed
to remove the 20,000 pound per hour from the system once
stabilization of the inert atmosphere has occurred.
[0058] In accordance with one embodiment of the invention, the
condenser 214 has two inlets and two outlets. One inlet is
for that portion of the superheated vapor exiting the
torrefaction chamber in duct 222. The second inlet is for the
cold water used to cool the superheated steam and to condense
out the excess water. One outlet is for the residual
superheated vapor which is now colder, no longer superheated,
and carrying less remaining water. The second outlet from the
condenser is for the cooling water which is now warmer due to
having been used to cool the superheated steam. The remaining
exiting residual vapor via duct 223 is re-heated via heat
exchanger 218 using heated exhaust from burner 212 to an
elevated temperature and then injected back into the
torrefaction chamber at any number of zones where it once
again can be used to heat the wood chips and to capture
evolved water from the wood chips. The superheated steam
provides the inert atmosphere within the torrefaction chamber.
The amount of the superheated vapor being supplied to the
condenser 214 versus the amount of the superheated vapor being
22
CA 02681282 2009-09-30
WYSSMONT 3.0-013
supplied to the burner 218 to be recycled can be controlled as
may be required by the water content and the water-handling
capacity of the burner.
[0059] By drying and torrefaction in one piece of equipment,
higher temperatures can be used in the drying section to
reduce the drying time. In the lower torrefying section 85,
temperatures between, for example, in the range of about 200 C
to about 290 C can be used. One advantage of drying and
torrefying in the same unit is that you do not have to control
the final moisture content from the drying section very
closely as you would with separate systems. In the rotating
tray type apparatus, the temperature and velocities of the
heating medium are easily adjusted.
[0060] In an apparatus where quenching occurs inside the unit,
the entire bottom of the torrefaction chamber (as part of it)
can be filled with water which is continuously replenished.
The hot torrefied cellulose material is wiped off the bottom
shelf into the water to prevent it from catching fire after it
leaves the apparatus. The hot torrefied material must be
cooled to a temperature where it will not combust in contact
with air, otherwise it can burst into flames. Quenching can
also be done in the discharge chute of the torrefaction
chamber or in other equipment that follows the torrefaction
chamber.
[0061] One advantage of the system is that drying and
torrefaction are done essentially under atmospheric pressure.
Another advantage is that carbon monoxide is maintained in the
system rather than outside as could occur with other drying
and torrefaction systems.
[0062] The design of the torrefaction chamber allows several
variables to be controlled: 1) The exit points from which the
23
CA 02681282 2009-09-30
WYSSMONT 3.0-013
superheated steam leaves the torrefaction chamber can be
designed to occur at one or more elevations simultaneously; 2)
The amount of superheated steam removed from the torrefaction
chamber at each exit point can be controlled; 3) The amount of
water removed from the superheated steam can be controlled; 4)
The re-injection temperature of the superheated steam and the
location of the re-entry points along the surfaces of the
torrefaction chamber can be controlled individually; 5) As the
product progresses further down through the torrefaction
chamber the superheated steam begins to include more and more
volatile gases; and 6) Superheated vapor containing volatiles
can be removed from one or more locations along the vertical
side of the dryer. As before, the superheated vapor can be
treated to remove excess water and/or volatile components.
[0063] In the lower sections of the torrefaction chamber there
is very little excess steam present since the wood chips are
no longer giving up excess water. The unique zone design of
the torrefaction chamber allows the collection of volatile
components to occur most efficiently in the lower sections
without the need to remove large amounts of water from the
volatiles. The torrefaction chamber system may include
numerous discharge points from which superheated steam can be
exited and then treated to remove water vapor and/or volatiles.
The chamber may also include numerous re-entry points into
which superheated steam can be re-injected into the
torrefaction chamber at controlled temperatures. By combining
both the drying process and the torrefaction process in a
single vertical design, the torrefaction process can be
designed to produce a torrefied wood product having maximum
caloric value and commercial value. The ability to precisely
control both the drying conditions and the torrefaction
24
CA 02681282 2009-09-30
WYSSMONT 3.0-013
conditions provides the advantage whereby the lignin present
in the cellulosic material is neither broken down nor driven
off.
[0064] The torrefaction chamber as thus far disclosed can be
zoned into multiple drying/torrefaction zones. This can be
done by physically dividing the horizontal cross section into
compartments using horizontally mounted baffle(s) or by design
of the fans so that temperature zones are created by the
segmented nature of the fan design. It is contemplated that
one can use a combination of these two methods.
[0065] Drying conditions are controlled by regulating the
temperature of each drying/torrefaction zone. There are
several possible ways of doing this. One contemplated example
includes the total caloric input into the system is determined
by the external burner firing rate. Heat recovery devices such
as condensers and air pre-heaters are used to recover "waste"
heat and to reduce the energy input into the system. In
another example, individual zones are created and controlled
by: regulating the volume of steam introduced into each zone;
by regulating the temperature of the steam introduced into
each zone; by regulating the vertical height of each zone; by
regulating the vapor takeoff from each zone; by regulating the
temperature at which the steam is condensed; by regulating the
temperature at which the volatiles are condensed; and by any
combination of the foregoing.
[0066] By way of example, an input of approximately 1500 BTU's
per pound of wood is contemplated to produce a torrefied
product having a usable caloric value of approximately 10,000
BTU's per pound. This ratio is impacted by the initial
moisture content of the cellulosic material and by the species
of cellulosic material being torrefied.
CA 02681282 2009-09-30
WYSSMONT 3.0-013
[0067] Pelletization of the torrefied wood allows the
torrefied wood to be used in commercial applications.
Processes that produce a product with less lignin result in
pellets that are more sensitive to breakage, create dust
problems at the point of usage, and have less commercial value.
The lignin in the product produces a strongly hydrophobic
material. This is desireable because prior to use as a fuel,
the torrefied, pelletized, cellulosic material will be stored
in the open and is exposed to climatic conditions. The more
hydrophobic the torrefied, cellulosic, material the less
likely the pellets will absorb water in high humidity
conditions, or will absorb water during a rain or snow. Any
water absorbed reduces the commercial value of the torrified
material by reducing the net caloric value. The process
produces an end product that is more uniform compared with
products produced by other processes. The end product is both
uniform, particle-to-particle, but also throughout the cross-
section of each particle. A uniform product allows for precise
and reproducible pelletizing of the torrified cellulosic
material.
[0068] Although the invention herein has been described with
reference to particular embodiments, it is to be understood
that these embodiments are merely illustrative of the
principles and applications of the present invention. It is
therefore to be understood that numerous modifications may be
made to the illustrative embodiments and that other
arrangements may be devised without departing from the spirit
and scope of the present invention as defined by the appended
claims.
26
